1. Field of the Invention
The field of the invention relates to relays and circuit breakers generally, and more particularly to certain new and useful advances in power system protective relays and circuit breakers of which the following is a specification, reference being had to the drawings accompanying and forming a part of the same.
2. Discussion of Related Art
Low-voltage power distribution systems are expected to deliver reliable power within constraints including, but not limited to, cost and size using available technology. Protective devices are chosen, installed, and adjusted to quickly operate, and selectively and reliably protect the low-voltage power distribution system. Time-based coordination and protection is the conventional basis for coordinating low-voltage power distribution systems.
It will be understood that commonly used terms such as sub-main, feeder, sub-feeder, and branch are used to refer to circuit lines and their respective protective devices. These terms are sometimes used somewhat interchangeably depending upon, for example, the number of circuit protection hierarchy layers or other factors. For example, for a simple electrical distribution system with 3 hierarchy layers of circuit protection, the circuit breakers providing protection may be referred to, from upstream to downstream, as main, feeder, and sub-feeder circuit breakers. Alternatively, the same devices may also be referred to as main, sub-main, and feeder circuit breakers, or optionally, main, feeder, and branch circuit breakers. For purposes of clarity, a protective device hierarchy designation, from upstream to downstream, of main, sub-main, and feeder circuit breakers shall be used herein to refer to the devices in a 3 layer protective system. In general, the term feeder circuit breaker shall refer herein to the lowest downstream circuit breaker in a system branch. Additionally, the term “tie” circuit breaker generally refers herein to a protective device on a line of a circuit that links two circuit branches or buses.
Conventional electrical distribution systems may have one or more layers of protective devices, each protective device defining a zone of protection. The zone of protection for each protective device extends downstream on one or more buses, to the next layer downstream protective device. Additionally, a tie circuit breaker may optionally be located on any line connecting two buses within a given zone of protection.
Circuit breakers providing the first layer of protection in an electrical system are generally referred to as “main” circuit breakers and define a first zone of protection extending from the main circuit breaker to the next one or more subsequent downstream circuit breaker providing a 2nd layer of protection. A circuit breaker providing the 2nd layer of protection (excluding a tie circuit breaker), is generally referred to herein as a “sub-main” circuit breaker, and defines a second zone of protection extending from the sub-main circuit breaker to the next one or more subsequent downstream circuit breaker providing a 3rd layer of protection, generally referred to herein as a “feeder” circuit breaker.
In the event of a fault (such as, but not limited to a short circuit, ground fault or an overload) the conventional protection system selectively coordinates the upstream and downstream breakers so that the nearest downstream breaker will clear the fault before the upstream breaker opens such that a minimal number of lines or feeders are de-energized.
Such time-based coordination can achieve good system selectivity; however, this system selectivity is achieved at the cost of speed for some of the coordinated relays. In a large power system, important main devices (i.e. upstream, source side protective devices such as main circuit breakers) may be significantly delayed to allow time for downstream layers of load side devices to clear selectively. To improve upon time-based coordination methods, zone-selective-interlocking (ZSI) techniques are often used.
A conventional zone selective interlock (ZSI) system improves the selective coordinated system by allowing the upstream breaker to identify a fault within its zone (i.e., ahead of the next layer of downstream circuit breakers or outlets) and clear this fault without adding the time delay required by selective-protection coordination.
More specifically, in a selectively coordinated protective system with conventional ZSI, when a downstream breaker detects a current greater than its ground fault (GF) pick-up, short time (ST) pick-up or its instantaneous (I) pick-up, it will send a restraint signal back to the upstream breaker. The upstream breaker, upon seeing the restraint signal, will begin to time out based on its conventional selective-coordination GF or ST time-delay-trip setting. In a first scenario, if the downstream breaker operates properly it will trip, thereby clearing the fault. Further, the upstream breaker will stop timing its GF or ST time-delay-trip setting and, thus, will not trip. In this first scenario, the downstream breaker cleared the fault and a minimal number of feeders were affected.
In a second scenario, if the downstream breaker detects the fault and sends a restraint signal to the upstream breaker but the downstream breaker does not operate properly to clear the fault the GF or ST time-delay-trip setting on the upstream breaker will time out and the upstream breaker will trip thereby clearing the fault. Thus, the upstream breaker acts as a back up breaker to the downstream breaker in the event that the downstream breaker does not operate properly. In this second scenario, however, all feeders downstream from the tripped upstream breaker are de-energized. In a third scenario, if the upstream breaker with conventional ZSI detects a GF or ST fault and does not receive a ZSI restraint signal from a downstream breaker, the upstream breaker will assume that the fault is in its protection zone (ahead of the next layer of downstream circuit breakers or outlets) and will trip with minimal time delay thereby quickly clearing the fault.
Patented disclosures of zone-selective interlock techniques may be found, for example, in U.S. Pat. Nos. 4,468,714; 5,151,842; 6,297,939 and US patent Publication 2008/0198521A1.
In certain cases, the conventional ZSI technique unnecessarily delays the tripping delay of a main circuit breaker. For example, a fault detection and isolation difficulty arises in the case of an electrical system fed by a first and a second separate power sources in parallel, and protected by a first and a second main circuit breakers connected in parallel, wherein a fault occurs between the first power source and its respective downstream main circuit breaker. If a tie circuit breaker is connected downstream and between the two main circuit breakers, and is CLOSED or conducting, the fault will be fed from both main circuit breakers. Additionally, if feeder circuit breakers downstream of the main circuit breakers are protecting motors, for example, the fault may also be fed from the motor loads. In such a case, mere knowledge of the fault current magnitude is not sufficient to identify the fault location, even when a conventional ZSI technique is used. In this case, the conventional ZSI operation would trip the feeder breakers and delay tripping of the main circuit breakers. Consequently, the clearing time for the fault increases, and in some cases results in nuisance tripping of circuit breaker in a non-faulted part of the system. Ideally, in this case, the main circuit breaker closest to the fault should trip to isolate the fault from the second power source without intentional delay. The tie circuit breaker as well as the second main circuit breaker should preferably remain closed. To accomplish this, the location of the fault must be known.
To overcome the above shortcomings of conventional ZSI, a Directional Zone Select Interlock (DZSI) technique was developed. For example, U.S. patent application Ser. No. 11/618,175 describes a DZSI technique and is assigned to the assignee of the present application and incorporated by reference herein.
However, in some scenarios, the DZSI algorithm may also fail to correctly identify the location of fault. Two such cases are discussed below. In each case the current is above ST pickup levels of at least one of the circuit breakers in the system.
DZSI and Motor Contribution:
FIG. 1 depicts a line diagram of a Directional Zone-Selective Interlocking system electrical power distribution system 11 having four ZSI zones, designated as protective Zone 1a, Zone 1b, Zone 2a, and Zone 2b. As shown, a first main circuit breaker M1 serves as an upstream breaker to downstream breakers in both Zone 1a and Zone 2a, which include a first sub-main circuit breaker SM1 and a tie circuit breaker T1 in Zone 1a, and feeder breakers SF1 and SF2 in Zone 2a. Similarly, a second main circuit breaker M2 serves as an upstream breaker to downstream breakers in both Zone 1b and Zone 2b, which include a second sub-main circuit breaker SM2 and the tie circuit breaker T1 in Zone 1b, and feeder breakers SF3 and SF4 in zone 2b. The tie circuit breaker T1 selectively connects a first bus B1 and a second bus B2. As shown, the electrical load downstream of the sub-feeder circuit breakers SF1 and SF2 are motor type loads.
FIG. 1 also depicts a bus fault at Zone 1b. Tie circuit breaker T1 between first bus B1 and second bus B2 is CLOSED (conducting), and second main circuit breaker M2 is OPEN (not conducting). Thus, the first main circuit breaker M1 will feed, or conduct, fault current into the second bus B2 through the tie circuit breaker T1. Additionally, if feeder circuit breaker SF1 and SF2 are connected to motor loads, they also will contribute to the fault current through the tie circuit breaker T1. For, example, if the first main circuit breaker M1 contributes 15 kA, and the feeder circuit breakers SF1 and SF2 each contribute 5 kA to the fault current, then the tie circuit breaker T1 will conduct a total fault current of 25 kA (i.e., 15 kA+5 kA+5 kA).
For the scenario illustrated in FIG. 1, the prior art Directional Zone Selective Interlock (DZSI) technique as described in U.S. patent application Ser. No. 11/618,175, titled Relay Device and Corresponding Method, calculates a Residual current (IR) for Zone 1a as a reference for determining the direction of currents carried by the feeder circuit breakers, where:IR=IM1*DM1+IT1*DT1 
Where IM1 & IT1 are currents in the first main circuit breaker M1 and the tie breaker T1 respectively, and DM1 & DT1 are relative current directions; and current into the zone is considered as positive while current out of the zone is considered as negative for calculation of current direction; givingIR=10 kA*(+1)+25 kA*(−1)IR=−15 kA
In the scenario of FIG. 1, the current carried by the first sub-main breaker SM1 is about −10 kA. Because the feeder circuit breakers SF1, SF2 are in ST pickup, and the residual current for Zone 1a is negative, IR=−15 kA, (i.e., out of Zone 1), the prior art DZSI technique considers this fault as a forward fault below Zone 2a and declares a feeder fault below Zone 2a. Actually, the fault is only present in Zone 1b and not in Zone 2a. Subsequently, the delay of the first main circuit breaker M1 and the tie circuit breaker T1 is incremented by about 200 milliseconds due to the DZSI operation. Although the prior art method correctly identifies a bus fault in Zone 1b, the tripping of the tie circuit breaker T1 is delayed because of the detection of sub-main fault in Zone 2a, resulting in increased fault clearing time.
Delayed Tripping of Sub-Main With DZSI:
Referring now to FIG. 2, the electrical distribution system of FIG. 1 is depicted, except, instead of a bus fault at Zone 1b, a fault is depicted downstream of the sub-main breaker SM1 in Zone 2a. The fault is fed current by the first main circuit breaker M1 as well as by feeder circuit breakers SF1, SF2. The first main circuit breaker M1 and first sub-main circuit breaker SM1 conduct currents in forward direction while both the feeder circuit breakers SF1 and SF2, being upstream of inductive motor loads, carry currents in the reverse direction. Since the conventional DZSI method algorithm does not consider the direction of currents in feeder breakers, the conventional DZSI method declares a feeder fault instead of sub-main fault. Consequently, the fault clearing time of first sub-main circuit breaker SM1 is delayed by about 100 milliseconds (ms).
A need exists for an improved system and method to implement directional zone-based protection to achieve fast fault protection while maintaining selectivity for a broad range of fault magnitudes, system configurations and load types. The present invention provides an improved method to overcome the deficiencies of the prior art.